全文获取类型
收费全文 | 736535篇 |
免费 | 5969篇 |
国内免费 | 1885篇 |
专业分类
化学 | 348753篇 |
晶体学 | 10237篇 |
力学 | 41808篇 |
综合类 | 19篇 |
数学 | 125396篇 |
物理学 | 218176篇 |
出版年
2021年 | 6206篇 |
2020年 | 6719篇 |
2019年 | 7688篇 |
2018年 | 20112篇 |
2017年 | 20025篇 |
2016年 | 20192篇 |
2015年 | 8467篇 |
2014年 | 13552篇 |
2013年 | 29337篇 |
2012年 | 26197篇 |
2011年 | 36432篇 |
2010年 | 25779篇 |
2009年 | 26121篇 |
2008年 | 31626篇 |
2007年 | 33339篇 |
2006年 | 23465篇 |
2005年 | 21864篇 |
2004年 | 20715篇 |
2003年 | 19382篇 |
2002年 | 18438篇 |
2001年 | 18414篇 |
2000年 | 14439篇 |
1999年 | 11109篇 |
1998年 | 9891篇 |
1997年 | 9639篇 |
1996年 | 9065篇 |
1995年 | 8039篇 |
1994年 | 8087篇 |
1993年 | 7893篇 |
1992年 | 8157篇 |
1991年 | 8712篇 |
1990年 | 8419篇 |
1989年 | 8321篇 |
1988年 | 8026篇 |
1987年 | 7872篇 |
1986年 | 7457篇 |
1985年 | 9351篇 |
1984年 | 9873篇 |
1983年 | 8327篇 |
1982年 | 8684篇 |
1981年 | 8124篇 |
1980年 | 7667篇 |
1979年 | 8422篇 |
1978年 | 8617篇 |
1977年 | 8645篇 |
1976年 | 8642篇 |
1975年 | 8244篇 |
1974年 | 8053篇 |
1973年 | 8294篇 |
1972年 | 6365篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
In this paper political and economic rationality is modelled regarding an economic problem with a four-dimensional dynamical system taking into consideration the information about the problem spread among the people who support the political alternatives. Under special parameter conditions velcro bifurcation occurs, which destabilizes the equilibrium points when information is going to spread. The last stable equilibrium point is related to the economically rational equilibrium point. 相似文献
993.
The form of the probability density derived from the evolution in time of a previously truncated frequency distribution of animal Liveweights is of interest in animal husbandry. Truncated frequency distributions arise when the heavier animals are sold for slaughter and the lighter animals retained. The demands of modern quality assurance schemes require that, given information on animal growth, the farmer is able to estimate the number of animals that would meet the specifications at some time in the future after truncation. Assuming that animal growth can be described by a linear stochastic differential equation, we derive an explicit expression for the probability density of animal Liveweights at any time after the truncation of an initial Gaussian density. It is shown that this probability density converges rapidly to a Gaussian density, so that after about 20 days of typical growth rates for lambs, the resulting density is practically indistinguishable from Gaussian. 相似文献
994.
For a graph G and a positive integer m, G(m) is the graph obtained from G by replacing every vertex by an independent set of size m and every edge by m2 edges joining all possible new pairs of ends. If G triangulates a surface, then it is easy to see from Euler's formula that G(m) can, in principle, triangulate a surface. For m prime and at least 7, it has previously been shown that in fact G(m) does triangulate a surface, and in fact does so as a “covering with folds” of the original triangulation. For m = 5, this would be a consequence of Tutte's 5‐Flow Conjecture. In this work, we investigate the case m = 2 and describe simple classes of triangulations G for which G(2) does have a triangulation that covers G “with folds,” as well as providing a simple infinite class of triangulations G of the sphere for which G(2) does not triangulate any surface. © 2003 Wiley Periodicals, Inc. J Graph Theory 43: 79–92, 2003 相似文献
995.
I. P. Kuranova K. M. Polyakov E. A. Smirnova W. E. Höhne V. S. Lamzin R. Meijer 《Crystallography Reports》2003,48(6):953-958
Crystals of Saccharomyces cerevisiae inorganic pyrophosphatase suitable for X-ray diffraction study were grown by cocrystallization of the enzyme with cobalt chloride and imidodiphosphate. Saccharomyces cerevisiae is a metal-dependent enzyme which catalyzes hydrolysis of inorganic pyrophosphate to orthophosphate. The three-dimensional structure of this enzyme was solved by the molecular-replacement method and refined at 1.8 Å resolution to an R factor of 19.5%. Cobalt and phosphate ions were revealed in the active centers of both identical subunits (A and B) of the pyrophosphatase molecule. In subunit B, a water molecule was found between two cobalt ions. It is believed that this water molecule acts as an attacking nucleophile in the enzymatic cleavage of the pyrophosphate bond. It was demonstrated that cobalt ions and a phosphate group occupy only part of the potential binding sites (two chemically identical and crystallographically independent subunits have different binding sites). The arrangement of ligands and the structure of the nucleophile-binding site are discussed in relation to the mechanism of action of the enzyme and the nature of the metal activator. 相似文献
996.
A crystalline quartz plate of thickness 1.070 mm is calibrated between 370 and 794 nm. Throughout this spectral interval, the retardance varied by 32π and the plate introduced quarterwave retardance 16 times at different wavelengths. The birefringence (ne−no) of crystalline quartz was calculated as a single quantity and varied from 0.00971 at 370 nm to 0.00891 at 794 nm. All measurements were carried out at 23°C. 相似文献
997.
Preliminary results on the mechanical, optical and electrical properties of composite gel electrolytes (CGEs) with fumed silica
(SiO2) as a filler added to gel polymeric electrolyte (GPE) based on PMMA, LiCF3SO3 and PC are presented in this paper. Added fumed silica is seen to enhance the mechanical properties of the GPE without changing
the conductivity significantly. The high ionic conductivity (×10−3 S/cm), high transmission in the visible region and nominal variance of conductivity and viscosity over a wide temperature
window show that these CGEs are potential electrolytes for electrochromic windows (ECWs). 相似文献
998.
V. I. Tsoy 《Optics and Spectroscopy》2003,94(4):607-612
In the framework of the matrix formulation of the coupled plane wave approach, a method is proposed for increasing the stability of the numerical analysis of vector electromagnetic fields in periodic structures, including dielectric and metallic anisotropic layers and relief boundaries. 相似文献
999.
The local vibrational spectra of copper crystals containing vacancies are calculated using the pair atomic potential derived in the framework of the resonance pseudopotential theory. The calculations are performed by a recursive method with due regard for the symmetry of the defect region. The frequencies of the vacancy-induced resonance vibrations of different symmetries are determined. 相似文献
1000.
L. M. Vasilyak M. N. Vasil’ev S. P. Vetchinin D. N. Polyakov V. E. Fortov 《Journal of Experimental and Theoretical Physics》2003,96(3):440-443
The action of an electron beam on ordered dust structures in glow and low-pressure RF discharges was studied experimentally. The electron beam produces destruction and dynamic displacement of the dust structure. In the center of a dust structure, an electron beam with a low electron energy (tens of eV) at currents up to 1 mA caused structural disordering and “melting” in the region of its action but did not excite external crystal regions. Local action of an electron beam with a high electron energy (25 keV) and a beam current above 10 mA caused deformation of the whole dust structure and shifted it in the horizontal direction so that it was carried away from the RF discharge zone. The effect of dust structure displacements can be used to locally remove particles from a plasma. 相似文献